1. Field of the Invention
This invention relates to a bar code reading optical apparatus.
2. Description of the Related Art
Conventional bar code reading systems have been made by assembling individually prepared parts including a light source (for example, a semiconductor laser) and an optical detector (for example, a photodiode). Therefore, they inevitably had dimensions determined by sizes of the parts, and could not be miniaturized beyond the limit imposed by the parts. Moreover, although assembled parts of conventional bar code reading systems need relative positional adjustment of a high accuracy smaller than 10 .mu.m, the positional relation among respective parts is liable to change with time after assembly, and such a system is not fully reliable.
A proposal to overcome the problem is a bar code reading complex apparatus in form of hybrid integration of a laser chip, a prism and other elements on a photodiode IC as shown in FIG. 1.
In the bar code reading complex optical apparatus shown in FIG. 1, mounted on a photodiode IC 101 are a prism 102 covering its photodiode PD' and a block 103 carrying a semiconductor laser 104 and located adjacent the prism 102. The prism 102 has a slope plane 102b angled by 45.degree. from its bottom place 102a. Formed on the slope plane 102b is a reflective film 105 of a dimension fully covered by a spot made by emitted light L' from a semiconductor laser 104 on the slope plane 102b.
In the bar code reading complex optical apparatus, the emitted light L' from the semiconductor laser 104 is reflected by the reflective film 105 on the slope plane 102b of the prism 102, and then converged by a converging lens (not shown) onto a bar code to be read. The signal light from the bar code returns to the bar code reading complex optical apparatus through the converging lens, then enters the photodiode PD', and is converted into an electrical signal there. Thus, the bar code is read out.
Although the bar code reading complex optical apparatus shown in FIG. 1 permits miniaturization, reduction in weight, omission of the job for positional adjustment, and increases the reliability, it involves some problems shown below.
In order to elevate the level of the signal light from the bar code, the numerical aperture of the converging lens for the signal light is made larger than the numerical aperture of the converging lens for the emitted light L'. In this respect, the bar code reading complex optical apparatus shown in FIG. 1 relies on making the reflective film 105 on the slope plane 102b of the prism 102 sufficiently small than the spot of the emitted light L' on the slope plane 102b. Therefore, part of the emitted light L' not reflected by the reflective film 105 does not contribute to scan beams. That is, the efficiency of use of the emitted light L' was low in the proposed system. In other words, the proposed system required a large amount of power for the semiconductor laser 104 to obtain scan beams of a given intensity. Moreover, not only unuseful is the part of the emitted light L' from the semiconductor laser 104 not reflected by the reflective film 105, but also obstructive is it as producing stray light causing a noise.
Considering that the use of the reflective film 105 needs a high cost to make it on the slope plane 102b of the prism 102 and needs a complicated job for establishing and maintaining relative positional accuracy between the semiconductor laser 104 and the reflective film 105, it may be possible to use a bifocal lens or a hologram having both an aperture for emitted light and another aperture for signal light in lieu of the reflective film 105. Then, the stray light problem can be greatly improved, but the efficiency of use of the emitted light L' from the semiconductor laser 104 still remains low. Moreover, when the bifocal lens or the hologram is used, part of the emitted light L' from the semiconductor laser 104 other than the part contributing to scan beams makes an undesired spot called garbage spot. This is shown in FIG. 2 in which numeral 107 refers to the bifocal lens, 108 to a scan spot, and 109 to a garbage spot.